Radiographic intensifying screen

ABSTRACT

A metallic intensifying screen having a grooved or indented surface to be disposed adjacent to radiographic film for enhancement of the yield of secondarily emitted particles thereby reducing film exposure time.

United States Patent [72] Inventor Joseph F. Tlnney Livennore, Calif.

[2!] Appl. No. 759,361

[22] Filed Sept. 12,1968

[45] Patented June 8, 1971 [73] Assignee The Bendix Corporation [54] RADIOGRAPHIC INTENSIFYING SCREEN [56] References Cited UNITED STATES PATENTS 2,740,900 4/1956 Ruble et al 250/83.6 2,943,206 6/1960 McGee et al. 250/80X 3,344,276 9/ l 967 Balding 250/80 Primary Examiner-Ralph G. Nilson Assistant Examiner-Davis L. Willis Attorney-Plante, Hartz, Smith & Thompson ABSTRACT: A metallic intensifying screen having a grooved or indented surface to be disposed adjacent to radiographic film for enhancement of the yield of secondarily emitted particles thereby reducing film exposure time.

PATENTEU JUN 8m 35 4315 FELL.

, 2 INVENTOR.

RADIOGRAPHIC INTENSIF'YING SCREEN BACKGROUND OF THE INVENTION 1. Field of the Invention Radiography, and more particularly, radiographic intensifying screens.

2. Description of Prior Art It is a common expedient in radiography to place a thin sheet of lead having smooth sides adjacent to radiographic film to first receive incident radiation. The interactions between the incident radiation and the lead atoms provide secondarily emitted particles to which radiographic film is highly responsive thereby reducing the time required to fully expose the film. In effect, the lead screen appears to "intensify" the incident radiation. Hence, the sheets of lead are commonly referred to as intensifying screens. They further have the desirable effect of blocking scattered radiation which would otherwise blur the image. It will be appreciated then that intensifying screens are widely used in the art.

SUMMARY OF THE INVENTION The present invention provides a new metallic intensifying screen which still further reduces the time required to fully expose radiographic film. The screen provides this reduction in its exposure time by emitting secondarily emitted particles at a higher rate than the rate associated with prior art intensifying screens. It will be appreciated that the degree of exposure of radiographic film is a function of the quantity of radiation to which it has been subjected. Therefore, a high incident rate of radiation will provide full exposure in a lesser period of time. A metallic intensifying screen according to this invention is characterized by having the side dispose adjacent to the radiographic film provided with a plurality of closely-spaced surface portions which are oblique to the nominal plane of the screen. For example, grooving or indenting these sides will provide these surface portions. It is believed that these oblique surface portions permit the escape of a higher percentage of particles secondarily emitted within the screen than that percentage obtained with prior art screens and furthermore tend to direct these escaped particles toward the radiographic film. As a result, a greater number of secondarily emitted particles reach the radiographic film within a given period of time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of an intensifying screen according to the present invention having a plurality of grooves being shown in combination with radiographic film.

FIG. 2 is a schematic illustration of another intensifying screen according to this invention having an indented surface to be disposed adjacent to radiographic film.

FIG. 3 is a schematic illustration of the screen of FIG. I having an electroluminescent coating on its grooved surface.

FIG. 4 is a schematic illustration of the screen of FIG. I having a secondarily emissive coating on its grooved surface.

FIG. 5 is a schematic illustration DESCRIPTION the screen of FIG. 1 having a secondarily emissive coating in combination with an electroluminescent coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, an intensifying screen according to the present invention is shown in combination with a sheet of radiographic film 12. The screen 10 may be constructed of a sheet of any metal having at least 13 electrons per atom, for example, lead, uranium, tantalum, tungsten, gold, silver, osmium and iridium. Lead is preferred since it is low in cost and readily available. The screen 10 is provided with a plurality of grooves 14 in its side disposed adjacent to the radiographic film 12. In practice, 40 grooves per inch, being 0.0l0 of an inch in depth, has proven satisfactory. As can be seen in the drawings, the grooves provide a plurality of closely-spaced surface portions 16, being in this case the sides of grooves 14, which areoblique to the plane of the screen 10. As can be seen in the drawings, these oblique surface portions 16 obliquely face the radiographic film 12 thereby tending to direct secondarily emitted electrons toward the radiographic film 12. To illustrate this effect, radiation photon 17 is shown entering the screen 10. The photon 17 has passed through the screen material to a location 13 within the screen 10 and has caused an atom at location 13 to emit an electron 23 along a path 25. It will be appreciated that the groove 14 has provided an air path for the electron 23. If, for example, a prior art screen without grooves were used, the electron would need to traverse substantially more screen material to reach the photographic plate 12 and would consequently have a lesser change of reaching the film 12 before absorption in the material of 10. As a further illustration of this effect, a radiation photon 18 is shown entering the screen 10. The photon 18 has passed through the screen material to a location 20 within the screen 10 and has thereby caused at atom at location 20 to emit an electron 22 along path 24. Generally, photons tend to produce electrons which are emitted in a direction which is oblique to the direction of travel of the photon. This is particularly true in the case of low energy photons. Accordingly, electron 22 has been depicted as proceeding along the path 24 which is oblique to the path of photon 18. It can be seen from the figure that the groove 14 has permitted the escape of the electron 22 from the screen material. It can also be seen in the figure that the path 24 of electron 22 has carried the electron across the groove 14 to the opposing wall thereof. The electron 22 is shown striking the opposing wall at location 26 and thereafter being reflected towards the radiographic film 12. The reflection depicted at point 26 is commonly referred to as back scattering." Back scattering" is a well-known effect in which high energy particles or radiation photons are reflected through angles greater than with respect to their original direction of motion after interaction with an atom or group of atoms. Considering now the above-described illustration, the effect of the grooves 14 may be understood. The grooves 14 provide for the escape of secondarily emitted particles from the screen material and furthermore provide surface portions which direct the particles through back scattering" towards the radiographic film. In practice, the increase in the rate of secondarily emitted particles reaching the radiographic film was observed to be approximately 230 percent over the rate of emission exhibited by prior art intensifying screens.

It should be understood that the present invention is not limited to intensifying screens having grooved sides. Other methods are available for providing a plurality of closelyspaced surface portion s which are oblique to the plane of the screen. For example, an intensifying screen having indentations 114 as shown in FIG. 2 is also satisfactory. In FIG. 2 the indentations are shown exaggerated in size for the purpose of illustration only. It should be further understood that the oblique surface portions may be flat, arcuate, or of any other cross section.

It is important that the surface portions be closely-spaced so as to provide a homogenous image on the radiographic film. A significant spacing of the grooves or indentation will cause their pattern to be superimposed on the image produced on the film. It is also preferred that the pattern formed by the sure face portions be substantially uniform over the surface of the screen so that their intensifying effect does not noticeably vary over the area of the image. ,1

In FIG. 3 the intensifying screen 10 of FIG. 1 is shown hav-. ing a coating 30 of electroluminescent material on its grooved side. Materials having this property are commonly referred to as phosphors. Typical of the group of phosphors are calciumtungstate, cadmium-sulfide, cesium-chloride, and sodium; chloride. Secondarily emitted particles and photons leaving the intensifying screen 10 strike the electroluminescent layer 30 causing the material to luminesce, i.e., to emit visible light. The use of an electroluminescent layer 30 further reduces exposure time since radiographic film is exposed more rapidly when subjected to radiation in the visible light band than when exposed to X- or gamma rays. As a further advantage, electroluminescent materials are generally hard and therefore protect the delicate grooves from damage. To provide the greatest protection to the grooves and the shortest exposure time, it is preferred that the layer 30 completely fill the grooves as shown in FIG. 3. However, thinner coating may be used for particular applications.

In FIG. 4, the screen of FIG. 1 is shown having a coating of a secondarily emissive material. More particularly, the material 40 should have an emissivity rate of at least 1 particle per particle striking it. For example, some of the materials having this characteristic are cesium oxide reduced on a base of silver, molybdenum, copper, tungsten and silver. This coating further enhances the rate of emission of secondary particles from the intensifying screen. To illustrate this effect, consider a photon 42 entering the screen material and undergoing a photoelectric interaction at location 44 thereby creating a secondary electron 46 which has escaped the screen and struck the coating 40 on screen 10. The penetration of the coating 40 by the electron 46 has created two secondary electrons 47 and in addition the collision between the secondarily emitted electron 46 and the coating 40 has emitted three more secondary electrons 48 all of which are highly effective in exposing radiographic film. The grooves provide a particular advantage when secondarily emissive materials are used as a coating. It is known that sloping incidence of particles striking secondarily emissive material increases the probability of copious emission of secondary particles. To take advantage of this effect, the groove angles may be adjusted to provide sloping incidence of the particles striking the secondarily emissive material, thereby optimizing the yield of particles emitted from the coating. As a still further refinement, a luminescent material 30 can be used as an outer layer over the secondarily emissive coating 40 as shown in FIG. 5 to further reduce exposure time.

The metallic intensifying screen according to this invention may be made at little, if any, increase in cost over that of conventional screens. MOREOVER, THE SCREENS according to this invention are readily adaptable to all applications in which conventional screens have been used.

Although this invention has been disclosed and illustrated with reference to a particular application, the principles involved are susceptible to numerous other applications which will be apparent to persons skilled in the art. The invention is therefore to be limited only as indicated by the scope of the appended claims.

Having thus described my invention, I claim:

1. A radiographic intensifying screen including a sheet of material having at least 13 electrons per atom having one side adapted to be disposed adjacent to an electron sensitive device for providing secondarily emitted particles when said sheet is subjected to radiation, an improvement comprising:

said one side having a plurality of closely spaced surface portions which are sufficiently oblique with respect to the plane of said sheet to substantially enhance the yield of secondarily emitted particles.

2. A radiographic intensifying screen of claim 1 comprising a material selected from the group consisting of lead, uranium, tantalum, tungsten, gold, silver, osmium and iridium.

- 3. A radiographic intensifying screen of claim 1 having surface comprising a plurality of closely-spaced grooves.

4. A radiographic intensifying screen of claim 1 having said surfaces comprising a plurality of closely-spaced indentations.

5. A radiographic intensifying screen of claim 1 and,

- a coating on said surface of a material which luminesces when struck by photons or electrons.

6. A radiographic intensifying screen of claim 5 wherein said coating is selected from the group consisting of calciumtungstate, cadmium-sulfide, cesium-chloride and sodiumchloride.

7. A radiographic intensifying screen of claim l and, a coating on said surface comprising a material exhibiting an average secondary emission of at least one particle per particle striking or penetrating said material.

8. A radiographic intensifying screen of claim 7 wherein said coating is comprised of a material selected from the group consisting of cesium oxide reduced on a base of silver, molybdenum, copper, tungsten and silver.

9. A radiographic intensifying screen of claim 7 and,

a coating on said secondary emissive material which luminesces when struck by photons or electrons.

10. A radiographic intensifying screen of claim 8 and,

a coating on said secondary emissive material selected from the group consisting of calcium-tungstate, cadmium-sulfide, cesium-chloride and sodium-chloride.

11. A radiographic system comprising:

a sheet of material having at least 13 electrons per atom for providing secondarily emitted particles when subjected to radiation, said sheet having one side provided with a plurality of closely spaced surface portions which are sufficiently oblique with respect to the plane of said sheet to substantially enhance the yield of secondarily emitted panicles; and

an electron sensitive device disposed adjacent said one side of said sheet for receiving said secondarily emitted particles.

12. A radiographic intensifying screen of claim 1 wherein said plurality of surface portions obliquely face said electron sensitive device.

13. A radiographic intensifying screen of claim 11 wherein said plurality of surface portions obliquely face said electron sensitive device. 

1. A radiographic intensifying screen including a sheet of material having at least 13 electrons per atom having one side adapted to be disposed adjacent to an electron sensitive device For providing secondarily emitted particles when said sheet is subjected to radiation, an improvement comprising: said one side having a plurality of closely spaced surface portions which are sufficiently oblique with respect to the plane of said sheet to substantially enhance the yield of secondarily emitted particles.
 2. A radiographic intensifying screen of claim 1 comprising a material selected from the group consisting of lead, uranium, tantalum, tungsten, gold, silver, osmium and iridium.
 3. A radiographic intensifying screen of claim 1 having surface comprising a plurality of closely-spaced grooves.
 4. A radiographic intensifying screen of claim 1 having said surfaces comprising a plurality of closely-spaced indentations.
 5. A radiographic intensifying screen of claim 1 and, a coating on said surface of a material which luminesces when struck by photons or electrons.
 6. A radiographic intensifying screen of claim 5 wherein said coating is selected from the group consisting of calcium-tungstate, cadmium-sulfide, cesium-chloride and sodium-chloride.
 7. A radiographic intensifying screen of claim 1 and, a coating on said surface comprising a material exhibiting an average secondary emission of at least one particle per particle striking or penetrating said material.
 8. A radiographic intensifying screen of claim 7 wherein said coating is comprised of a material selected from the group consisting of cesium oxide reduced on a base of silver, molybdenum, copper, tungsten and silver.
 9. A radiographic intensifying screen of claim 7 and, a coating on said secondary emissive material which luminesces when struck by photons or electrons.
 10. A radiographic intensifying screen of claim 8 and, a coating on said secondary emissive material selected from the group consisting of calcium-tungstate, cadmium-sulfide, cesium-chloride and sodium-chloride.
 11. A radiographic system comprising: a sheet of material having at least 13 electrons per atom for providing secondarily emitted particles when subjected to radiation, said sheet having one side provided with a plurality of closely spaced surface portions which are sufficiently oblique with respect to the plane of said sheet to substantially enhance the yield of secondarily emitted particles; and an electron sensitive device disposed adjacent said one side of said sheet for receiving said secondarily emitted particles.
 12. A radiographic intensifying screen of claim 1 wherein said plurality of surface portions obliquely face said electron sensitive device.
 13. A radiographic intensifying screen of claim 11 wherein said plurality of surface portions obliquely face said electron sensitive device. 